> The email address is added to the product by hand in a custom field

I supouse that the field for entering email, is an email field, if so:
then in your rule add a condition: [your_email_field] is not empy, if that condition is far away to be implemented in rules: condition, then use [rules conditional module][1], when your a ready to implement 'send emal' in your rule, add condition: [your_email_field] is not empty, if user entered a 'fake email', drupal will send email, but it will not go anywhare, this way you can avoid your error, and only valid emails will be delivered.


  [1]: https://www.drupal.org/project/rules_conditional